Recent developments in telecommunications engineering have led to digital telecommunication systems which provide a conversion of speech signals into digital signals in a subscriber station for telephone subscribers and in which a digital telephone connection by way of a standard digital communication channel with a bit rate of 64 kbits/s (per transmission direction) can be completed between the subscriber station and the switching station, the subscriber station being directly connected to the digital telephone connection and, moreover, a signaling connection for outslot signaling with a bit rate of, for example, 8 kbit/s (per transmission direction) can permanently exist outslot of the 64 kbit/s communication channel, whereby, in addition to outslot signaling, inslot signaling can also be provided within the message channel (ISS'79, pp. 773-780). In addition to speech, texts, data and images can be transmitted in the 64 kbit/s channels of such a digital telecommunication system.
In such digital telecommunication systems, there is the possibility that specific subscriber locations, in terms of exchange orientation, do not belong to that switching center to which the appertaining subscriber station is directly connected but, rather, to a different switching center --via a permanently through-connected connection extending by way of the switching center. Such a network configuration can be provided, for example, in a services-integrated digital telecommunication system in which data subscriber stations are only indirectly connected to an appertaining data switching center via permanently through-connected connections extending through telephone switching centers (telefon-report 11 (1975), 3, 86-90), or also in the case of an outside (PBX) extension whose switching center is connected via a leased channel extending via the public telecommunication network.
Such a connection, which is only indirect, of a subscriber station to the appertaining switching center can lead to the fact that only the standard, actual message channel with a bit rate of 64 kbit/s is available on a part of the overall connection path, but not an additional signaling channel as well, so that identical communication possibilities and signaling operations and corresponding interface conditions are not possible, at least without further considerations for directly-connected and for indirectly-connected subscriber stations. However, identical communication possibilities, signaling operations and interface conditions will be required, particularly given subscriber stations which are otherwise identical.